Transistor inductor



Sept. 19, 1967 R. E. ARSENEAU TRANSISTOR INDUCTOR Filed Jan. 24, 1964 mm m F \J \V W B| I T l mmm WW Ill-M pupa DIODE-9 F IG 2 ATTORNEY UnitedStates Patent O 3,343,003 TRANSISTOR INDUCTOR Roger Edward Arseneau,Elkgrove Village, 11]., assignor to International Telephone andTelegraph Corporation Filed Jan. 24, 1964, Ser. No. 340,011 4 Claims.(Cl. 307-885) This invention relates to circuits for performingfunctions which are equivalent to the functions of an inductor, and moreparticularly to transistorized circuits for providing such functions.

An inductor has both desirable and undesirable characteristics. Amongother things, an inductor has desirable characteristics which may beused to provide a low D.C. impedance and a high A.C. impedance. Aninductor also provides a current lag which facilitates the design ofcertain circuits such as choke coils, filters, and the like. In additionto the desirable characteristics, an inductor also has undesirablecharacteristics which sometimes cause problems. For example, the designand construction of an inductor may become extremely critical whenconflicting demands are placed on it. To illustrate this point, considerthe requirements of a repeat coil or transformer for coupling a sourceof voice signals to an electronic switching network. If the transformerhas good voice transmission characteristics, current may occasionallyreach levels which cause the switching system to fail. On the otherhand, if the transformer passes a current having characteristics whichinsure proper switching, the quality of voice transmission maydeteriorate. Of course, it should be understood that this transformerexample is cited only to illustrate the problem. Many other examplescould also be cited to illustrate other problems which are encounteredwhen an inductor is used.

The functions of an inductor are similar to the functions of a flywheelsince they both add mass to maintain a steady state condition in akinetic energy system. For example, an inductor maintains a uniformdelivery of current despite non-uniform variations (an A.C. signal) involtage applied across the inductor. Thus, one of the most importantconsiderations in the design of an inductor is the provision of a propermass which is well matched to the needs of the system in which it isused. This matching is done by a selection of the physical dimensions ofthe inductor. It must be large enough to govern the electrical system inwhich it is used, and small enough not to overload the source whichpowers the system. Moreover, it also should be tuned for properoperations in response to characteristic variations of the kineticenergyof the electrical systems. Since the inductor characteristics dependupon its physical dimensions, it has been expensive to provide a greatnumber of in ductances having a wide variety of characteristics. Severeeconomic burdens result if a great number of different sized piece-partsmust be made and stocked in inventory.

In addition to mass considerations, there are other non-related designconsiderations which inherently occur in many electronic circuits.Therefore, a real economy results if an electronic circuit which isprimarily designed toprovide the functions of an inductor also providesa solution for some of the other design problems. For example, almostall circuits require amplification of signals. Thus, a circuit combininginductance and amplification would be quite valuable.

Accordingly, an object of the invention is to provide new and improvedcircuits having the electrical characteristics of an inductor. A moreparticular object is to provide a transistorized circuit having both aninductors characteristics and an amplification capability.

Yet another object of the invention is to provide new and improveddevices for coupling dissimilar circuits. Here an object is to eliminateexpensive and involved 3,343,003 Patented Sept. 19, 1967 circuitryrequiring critical components, especially transformers. In thisconnection, an object is to substitute an inexpensive capacitivecoupling for an expensive transformer coupling.

Still another object of the invention is to reduce the cost of designingand building circuits. In particular, an object is to reduce the cost ofelectronic switching telephone systems. Here an object is to accomplishthe above and other objects by a circuit which not only eliminatesproblems caused by inductor design considerations, but also providesamplification in an electronic switching network.

In accordance with one aspect of this invention, an electronic networkis provided with a single terminal which functions as both an input andan output. Connected thereto are a pair of electronic devices, bothhaving input, output, and control electrodes. These devices are coupledin a feed back loop relation so that one transistor controls the outputof the other transistor as a function of A.C. voltage variations at thesingle terminal. This control is accomplished in a manner such that theother transistor applies a constant DC. current to the terminal despiteA.C. variations appearing thereat.

The above mentioned and other features of this invention and the mannerof obtaining them will become more apparent, and the invention itselfwill be best understood by reference to the following description of anembodiment of the invention taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a schematic circuit diagram showing a transistorized circuitwhich has characteristics that are equivvalent to the characteristics ofan inductor; and

FIG. 2 is a schematic circuit diagram showing how the circuit of FIG. 1may be used in an electronic switching telephone system.

FIG. 1 discloses a preferred embodiment of the invention which is hereshown as a transistorized circuit having the electrical characteristicsof an inductor. The circuit is an electronic network with a singleterminal 50 which functions as both an input and an output. Connectedthereto are a pair of electronic devices, 51, 52, both of which haveinput, output, and control electrodes a, b, and 0 respectively. Ofthese, device 51 is here shown as a PNP transistor and device 52 as anNPN transistor, both operating in their active range, to function whenpositive current flows into terminal 50. If polarities are reversed tocause a negative current at terminal 50, the transistor types are alsoreversed. These devices are coupled in a feed back loop relation so thatone transistor controls the output of the other transistor as a functionof A.C. signal variations appearing at the single terminal 50. Thiscontrol is accomplished in a manner such that the other transistor.supplies a constant DC. current to the terminal. Thus, there is, ineffect a low impedance for DO. and a high impedance for A.C.

In construction, this circuit is arranged with both the emitter a andbase c of the PNP transistor 51 individually coupled to the input-outputterminal 50, the emitter a via a resistive element 53, and the base 0via a capacitive element 54. The emitter a is also connected to groundby a resistor 55 which helps control the impedance at point 50, andwhich cooperates with the resistor 53 to form a voltage divider. Thebase 0 is further biased to a quiescent point of operation by a voltagedivider 56, 58 connected between positive battery and ground. A resistor59 is coupled between a 12 volt source and the collector b of transistor51 and base 0 of transistor 52. This resistor 59 stabilizes thecollector current when the collector junction is reverse biased (i.e.the 1 The circuit works this way. The voltage divider 56, 58 normallyestablishes a potential at the base of the transistor 51 which is alittle less positive than the potential desired at the terminal 50.Thus, when current flows into terminal 50, transistor 51 is normallyforwardly biased to drive a control current into the base of thetransistor 52. The circuit values are such that the voltage drop acrossresistor 53 is enough to cause the transistor 52 to operate in itsactive region and conduct the current entering terminal 50 to ground G1while keeping the potential at terminal 50 more positive than thepotential at the base of transistor 51. The potential at the base oftransistor 51 is high enough to keep both of the transistors 51, 52 fromsaturating.

Means are provided for causing the circuit to present a low D.C.impedance and a high A.C. impedance. To understand how this occurs,assume that an A.C. signal at the point 50 is such that the incomingvoltage begins to increase. This increase in voltage is applied to oneend of both resistor 53 and capacitor 54. If this voltage change occursmuch faster than the RC time constant of the network of capacitor 54 andresistors 56, 58, the same change is applied to both the base andemitter of the transistor 51. Since both of these electrodes experiencethe same voltage variations, there is no change in the voltage acrossresistor 53, and, therefore, no change in the collector current oftransistor 51. Thus, the current driving into the base of transistor 52remains the same so that there is no change in its collector current. Ina like manner, the voltage across resistor 53 and the collector currentof transistor 52 remain constant when the potential at terminal 50decreases. It should now be apparent that since a change in voltage atterminal 50 does not produce a corresponding change in the current intoterminal 50, there is a very high impedance to the A.C. change.

The well known inductor causes a current lag. This effect is seen atterminal 50 where a current change also lags after a voltage change. Ingreater detail, for a very slow increase in potential at terminal 50,the charge on capacitor 54 increases, thus increasing the voltage acrossresistor 53. With these changes, the bias on transistor 51 iseffectively shifted to increase the collector current of transistor 51.This means that the current driving into the base of transistor 52increases to cause an increase in the collector current of transistor52. This increase in co1- lector current lags after the increase involtage at terminal 50 because it takes time after the voltage changeoccurs at terminal 50 before the charge on capacitor 54 can change.

For a change in the DC. level of the current at terminal 50, the voltageat terminal 50 increases just enough to increase the voltage across bothresistor 53 and capacitor 54. This causes an increase in the basecurrent to transistor 52 to cause it to increase its collector currentto terminal 50.

Means are provided for using these functions to cause electrical effectswhich are equivalent to changing the physical dimensions of an inductor.More particularly, for a given A.C. signal, no elfect is seen in thecurrent flow from point G1 through the emitter-collector of thetransistor 52 to the point 50; or stated another way, there is a lowD.C. impedance and a high A.C. impedance. If the undulations of the A.C.signal become more rapid, there are no significant voltage variations atthe base of the transistor 51, with respect to the voltage on itsemitter. As the undulations slow, the capacitor 54 comes closer toreceiving a full charge, and the voltage variations at the base of thetransistor 51 become more significant with respect to the voltage on itsemitter. If the undulations are slow enough, a significant A.C. signaleffect is felt in the output of the transistor 52. If the capacitor 54charges sufficiently, the voltage in the base circuit of the transistor51 changes by an amount which is adequate to produce a relatively greatchange in the current through the transistor 52. The answer to theproblem of what is rapid or slow is determined by the circuit valueswhich establish the RC. time constant. A large time constant causes aslower response. Thus, by varying the size of the capacitor, one getsresults which are the same as those which are obtained by varying thesize of the inductor. Thus, by selecting the value of capacitor 54,relative to the value of other components, the circuit may be given anoutput stimulating the output of any one of a great variety ofinductors.

In one exemplary construction, the following circuit values were usedwith good results.

Transistor 51 Type, 2N1309. Transistor 52 Type 2N1308. Resistor 53 3909.Resistor 55 39K.

Resistor 56 K. Resistor 58 10K. Resistor 59 100K. Capacitor 54 l0 mf.

With the circuit constructed from components having the above circuitvalues, it was found that a good inductive characteristic was achieved.from zero through 5,000 cycles per second. The circuit was designed tooperate over this range of frequencies because the transistor inductorcircuit was used as a substitute for a voice frequency choke coil.Obviously, the invention contemplates a selection of any appropriatecircuit values as may be required by the particular frequency band ofinterest for a given application.

Means are provided for giving a negative resistance which is comparableto amplifying the A.C. signal that appears at terminal 50. In greater.detail, an external circuit driving into terminal 50 sees an impedancewhich is fixed primarily by resistors 53, 55. By reducing the resistanceat 55, the impedance seen at terminal 50 may be raised to infinity, oreven higher into a negative impedance zone. Thus, the signal source (notshown in FIG. 1)

sees a smaller effective load which amounts to a signal.

amplification.

This negative impedance occurs because the resistors 53, 55 form avoltage divider connected between terminal 50 and ground. Here the ratioof the divider arms are selected so that the A.C. voltage variationsexperienced in the circuit of the emitter of transistor 51 are less thanthe A.C. signal caused voltage variations experienced in the circuit ofbase of transistor 51. When the potential raises at terminal 50, it alsoraises at the common tie point between the emitter of transistor 51, theresistor 53, and the resistor 55. This causes an increase in the currentflowing through the resistor 55. The potential across resistor 53remains constant because emitter a and base c experienced the same risein potential; thus, the current through resistor 53 also remainsconstant, and the increase in current through the resistor 55 must occurat the expense of a decrease in current into the emitter of transistor51. This decrease in emitter current causes a decrease of the currentdriving into the base of transistor 52. Therefore, there is a reduc-.tion in the collector current of transistor 52. Hence, an.

increase in the potential at terminal 50 causes a decrease in thecurrent into terminal 50, thereby producing an effect which is as ifthere were a reduction in impedance or more conventionally, a negativeimpedance appears at terminal 50.

With the'circuit values given above, it was found that the inputimpedance at terminal 50 could be varied from 2K through infinity to5009 by varying resistor 55 from 18K to 200K.

There are, of course, many .uses for the invention. One exemplary use isfound in an electronic switching telephone system shown in FIG. 2. Theprincipal divisions and a link 63. The subscriber line 60 terminates atone end in a telephone sub-set 64 .(which may be a calling subscriberstation), and at the other end in a repeat coil signal caused 65. Theswitching network 62 terminates at one end in the transistorizedinductor 61 and at the other end in a constant current source 66. Thecomponents 67 indicate another identical path to a called subscriberstation.

A low D.C. impedance high A.C. impedance path may be traced from groundG2 through inductor 61, network 62, and constant current source 66 tobattery B1. Thus, at capacitor 68, the speech path may be capacitivelycoupled from line 60 to network 62. This means that the repeat coil 65may be an inexpensive item of standard design. Its design does not haveany effect on DC. current flow through the network 62.

Heretofore, the DC. holding path for the network 62 extended through thesecondary winding of repeat coil 65. The network was subject tomalfunctions if the holding path was not controlled quite closely. Thesubscriber lines, such as 60, are notorious for their non-uniformcharacteristics. Thus, the design of the transformer 65 has been verycritical.

While the principles of the invention have been described above inconnection with specific apparatus and applications, it is to beunderstood that this description is made only by way of example and notas a limitation on the scope of the invention.

I claim:

1. An electronic network having a single input-output terminal, a pairof electronic devices connected to said terminal in alow-D.C.-resistance-high-A.C.-impedance configuration, one of saiddevices being connected to control the conductivity of the other of saiddevices, both of said devices having base, emitter, and collectorelectrodes, said one device having at least a capacitive elementconnected between the base and emitter electrodes, and means comprisingsaid two devices and responsive to a change of current into saidterminal for compensatingly changing the conductivity of said otherdevice to counteract the change in current into said terminal, wherebysaid network supplies a constant direct current to said terminal despitethe variations in an A.C. signal appearing at said terminal, wherein afirst of said devices is a PNP transistor and a second of said devicesis an NPN transistor.

2. The network of claim 1 wherein the emitter and base electrodes ofsaid PNP transistor are individually coupled to said terminal via aresistive element and said capacitive element respectively, and thecollector electrode of said PNP transistor is coupled to drive currentinto the base electrode of said NPN transistor, a potential point, and acircuit extending from said potential point through the emitter andcollector electrodes of said NPN transistor to said terminal, said lastnamed circuit being the path for supplying said direct current to saidterminal.

3. The network of claim 2 and a resistor coupled to cooperate with saidresistive element and form a voltage divider for establishing the inputimpedance at said terminal, the value of said resistor being selected toprovide a decrease in current flowing into said terminal responsive toan increase of voltage appearing at said terminal, said emitterelectrode of said PNP transistor being coupled to a point on saidvoltage divider.

4. The network of claim 2 wherein the Value of said capacitive elementis selected to provide a predetermined time lag after which said A.C.signal may cause a change in said direct current.

References Cited UNITED STATES PATENTS 2,892,164 6/1959 W011 30788.52,892,165 6/1959 Lindsay 307-885 3,001,157 9/1961 Sipress et al. 333-241X 3,152,309 10/1964 Bogusz et al. 307--88.5

ARTHUR GAUSS, Primary Examiner.

I. ZAZWORSKY, Assistant Examiner.

1. AN ELECTRONIC NETWORK HAVING A SINGLE INPUT-OUTPUT TERMINAL, A PAIROF ELECTRONIC DEVICES CONNECTED TO SAID TERMINAL IN ALOW-D.C.-RESISTANCE-HIGH-A.C.-IMPEDANCE CONFIGURATION, ONE OF SAIDDEVICES BEING CONNECTED TO CONTROL THE CONDUCTIVITY OF THE OTHER OF SAIDDEVICES, BOTH OF SAID DEVICES HAVING BASE, EMITTER, AND COLLECTORELECTRODES, SAID ONE DEVICE HAVING AT LEAST A CAPACITIVE ELEMENTCONNECTED BETWEEN THE BASE AND EMITTER ELECTRODES, AND MEANS COMPRISINGSAID TWO DEVICES AND RESPONSIVE TO A CHANGE OF CURRENT INTO SAIDTERMINAL FOR COMPENSATINGLY CHANGING THE CONDUCTIVITY OF SAID OTHERDEVICE TO COUNTERACT THE CHANGE IN CURRENT INTO SAID TERMINAL, WHEREBYSAID NETWORK SUPPLIES A CONSTANT DIRECT CURRENT TO SAID TERMINAL DESPITETHE VARIATIONS IN AN A.C. SIGNAL APPEARING AT SAID TERMINAL, WHEREIN AFIRST OF SAID DEVICES IS A PNP TRANSISTOR AND A SECOND OF SAID DEVICESIS AN NPN TRANSISTOR.